Certain radio frequency (RF) signal modulation techniques such as quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) suffer from the loss of signal power with increasing modulation levels. Such techniques also yield errors because of a low signal-to-noise ratio (SNR). Compensation for such errors requires bandwidth compression by an amount approximately equal to the square of the power increase. Modulation techniques such as frequency shift keying (FSK), Guassian minimum shift keying (GMSK) and QAM transmit non-return to zero (NRZ) line-coding offer bandwidth savings by concentrating the bandwidth around a carrier. Bi-phase coding serves to keep information sidebands away from the carrier. Further bandwidth savings can be achieved by transmitting on a single sideband.
U.S. patent application Ser. No. 09/623,776, filed on Sep. 8, 2000, in the name of Chandra Mohan et al., and assigned to Thomson Consumer Electronics, Inc. (incorporated by reference herein) describes a variable aperture coding (VAC) system for use in spread spectrum transmission that provides for multiple phase coding of an input NRZ bit stream to afford improved bandwidth compression. The VAC coding scheme disclosed in the Mohan et al. '776 application utilizes analog circuitry for generating the VAC signals that subsequently undergo band pass filtering by an Intermediate Frequency (IF) stage. Such band pass filtering tends to smear the edges of the original VAC signal resulting in a loss of control of the zero crossings.
Thus, there is a need for a technique for generating VAC signals that require no band pass filtering to convert to the carrier frequency, thus affording greater control over the zero crossings.